This invention relates to a stereoscopic image projection apparatus such as a color stereoscopic projection television set, and to a telecentric zoom lens for use in such apparatus.
Stereoscopic projection can be achieved by using light with one type of polarization to project an image for the left eye onto a screen, and light with another type of polarization to project an image for the right eye onto the same screen. The viewer wears eyeglasses that transmit light of the appropriate polarization to each eye, and thereby sees the image stereoscopically, as if it were three-dimensional.
A convenient way to project a polarized image is to employ a projector comprising a liquid-crystal panel and a projection lens. One prior-art color stereoscopic projection apparatus employs six such projectors, three for the three primary colors in the left-eye image, and three more for the three primary colors in the right-eye image. With this type of apparatus, however, aside from the problems of size, weight, and cost occasioned by having six projectors, there is the problem of image registration: the six images must be accurately positioned with respect to one another on the screen. This makes the apparatus difficult to set up and adjust.
Another prior-art stereoscopic color projection apparatus avoids these problems by employing two color liquid-crystal panels, one forming the left-eye image and the other forming the right-eye image, and combining the two color images by means of a polarizing beam splitter (functioning as a beam combiner) for projection through a single lens. With this type of apparatus, however, new problems arise. A color liquid-crystal panel employs red, green, and blue filters to produce the three primary colors, but these filters necessarily absorb at least two-thirds of the light: hence the projected image lacks adequate brightness. Moreover, only one-third of the pixels are available for each color: hence the projected image lacks adequate resolution.
Further problems relate to the projection lens. For a variety of reasons, which will be explained in the detailed description of the invention, a telecentric zoom lens is desirable, but such lenses are scarcely to be found in the prior art. Conventional zoom lenses suitable for use in projection display apparatus of the present type generally comprise four or five lens groups, making the structure of the lens barrel complex. They also require cam-driven compensators to maintain correct focus at different magnifications.